Article pubs.acs.org/cm
Swelling of Polymer Dielectric Thin Films for Organic-TransistorBased Aqueous Sensing Applications Eric Verploegen,†,‡ Anatoliy N. Sokolov,‡ Bulent Akgun,§,⊥ Sushil K. Satija,§ Peng Wei,‡ Daniel Kim,‡ Matthew T. Kapelewski,‡ Zhenan Bao,‡ and Michael F. Toney*,† †
Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States § Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States ‡
S Supporting Information *
ABSTRACT: Sensors based on organic thin-film transistors (OTFTs) are of considerable interest for chemical and biological detection applications, and the development of highly sensitive, chemically specific, low-cost sensors operating in aqueous environments will have a profound impact. However, the behavior of the dielectric and semiconducting thin films in OFTF-based sensors during underwater operation is not well understood. Here we investigate OTFT-based sensor materials, specifically a polymer dielectric film of cross-linked poly(4-vinylphenol) (x-PVP), used in OTFTs operating in aqueous environments. We show that immersing x-PVP thin films in a 90:10 water−methanol (model analyte) solution causes swelling of nearly 30% and a corresponding 300% increase in the film dielectric constant. Hence, to quantify the charge-transport behavior of organic molecules within aqueous environments, this drastic change in the capacitance must be accounted for in sensor material design. KEYWORDS: organic-field-effect transistors, dielectrics, polymeric materials, thin films
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INTRODUCTION The rapid development of the field of organic electronics has sparked great interest in the use of organic thin-film transistors (OTFTs) as low-cost flexible circuits,1−5 radio-frequency identification cards,6 electronic skin,7 and especially electronic sensors.8−10 This is largely due to their compatibility with flexible materials, simple, inexpensive fabrication methods, and the synthetic versatility of organic materials. OTFT-based sensors (Scheme 1c) are attractive for a broad range of chemical and biological detection applications,11,12 including environmental monitoring,13 detection of chemical warfare agents, in situ medical diagnostics,14−17 drug delivery, and food storage.18 The development of low-cost chemical sensors that can operate in aqueous environments with high specificity and sensitivity will have a profound impact on important issues such as maintenance of safe water supplies19 and disease detection and prevention.20,21 In addition to the need for organic semiconducting materials, flexible low-cost devices require thinfilm dielectric layers that are flexible and can be deposited using © 2013 American Chemical Society
solution-processing methods. One such material, poly(4vinylphenol) (PVP), can be spin-cast and subsequently crosslinked with 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (HDA) to form a stable polymer dielectric thin film (xPVP).22 This material has been used for OTFTs operating in aqueous environments.21,23,24 However, the behavior of the dielectric and semiconducting thin films during underwater OTFT operation has not been studied. Here we show that immersing x-PVP thin films in a 90:10 water−methanol solution results in swelling of nearly 30% and a corresponding 300% increase in the dielectric constant of the film. In order to quantify the charge-transport behavior of organic molecules within aqueous environments, this drastic change in the capacitance must be accounted for. Moreover, interaction of a desired analyte with the polymer dielectric should be Received: September 27, 2013 Revised: December 5, 2013 Published: December 6, 2013 5018
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Scheme 1. Structures of PVP and PVP−PS Random Copolymers (a and b, Respectively),a (c) OTFT Geometry, and (d) Geometry of the Capacitor for Dielectric Measurements
a
The ratio of n to m can be varied to tune the PVP-to-PS ratio. Information. The organic semiconductor 5,5′-bis(7-dodecyl-9Hfluoren-2-yl)-2,2′-bithiophene (DDFTTF) was synthesized according to previously reported methods.28 DDFTTF was purified by gradient sublimation under high vacuum twice before use. Dielectric Film Fabrication. We prepared solutions of 40 mg/mL PVP or the PVP−PS copolymer in propylene glycol monomethyl ether acetate (PGMEA). After the polymer was fully dissolved, 1 μL of triethylamine (TEA) was added per 1 mL of polymer solution, and the resulting solution was shaken until homogeneous. A second solution of 4 mg/mL 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (HDA) in PGMEA was prepared. This polymer−TEA solution was then combined with the HDA solution at a 1:1 ratio, shaken until homogeneous, filtered through a 0.2 μm filter, and spin-cast onto a ntype silicon substrate (purchased from Wafer Works Corp.27 with a resistivity between 0.002 and 0.004 Ω) at 4000 rpm for 60 s. The thin films were then heated for 1 h on a hot plate at 110 °C to cross-link the thin films. Neutron Reflectivity (NR). NR measurements were performed on the NG-7 horizontal reflectometer at the Center for Neutron Research at the National Institute of Standards and Technology (Gaithersburg, MD). The instrument had a wavelength (λ) = 0.4768 nm and a wavelength spread (Δλ/λ) = 0.025. A schematic of the reflectivity geometry utilized is illustrated in Figure S1 in the Supporting Information. Measurements performed as a function of θ define the scattering wave vector (q) normal to the film, q = 4πλ−1 sin θ. The uncertainty in the reflectivity data was calculated from the counting statistics. The wafers used for these measurements are 8 mm thick, in order to allow the neutron beam to enter the side of the wafer at the highest incident angles used in this study. The thin films were immersed in water or 90:10 water−methanol solutions for >3 h before the start of the measurement in order to ensure the films had reached an equilibrium swelling state. We used the MOTOFIT29 package for Igor (commercially available from WaveMetrics, Inc.27) for fitting of the reflectivity profiles. Details on the fitting procedure used are provided in the Supporting Information. Device Fabrication and Characterization. For all devices, the top gold contacts were thermally evaporated in high vacuum at a rate of 1 Å/s while the substrate holder was rotated. The electrode dimensions for capacitor measurements were defined with a circular shadow mask of ∼2.2 or 14.1 mm2. Devices for capacitance measurements were fabricated by affixing a 20 μm gold wire to each electrode with conductive silver paste (Ted Pella Inc., LeitSilber27). The capacitance measurements, before and after immersion in water or a water−methanol solution, were taken using an Agilent E4980A Precision LCR meter.27 Capacitances were measured at 1 kHz frequency with a 0.01 V alternating-current signal to provide an optimal signal-to-noise ratio.23 The DDFTTF films were deposited by thermal evaporation (Angstrom Engineering, Inc.27) at a rate of 0.1− 0.2 Å/s under a pressure of 5.0 × 10−7 Torr. The substrate temperature for DDFTTF deposition was kept at 60 °C and controlled by heating a copper block during deposition. The electrode dimensions for transistors were defined by a shadow mask with channel width (W) and length (L) of 4 mm and 50 μm, respectively. The electrical measurements of OTFT devices were carried out in
considered to achieve both an accurate sensor performance as well as a mechanistic understanding of the sensor mechanism. These conclusions will significantly impact the design of polymer dielectrics for organic transistors and sensors. For the successful development of OTFTs that can detect desired analytes in aqueous environments with high sensitivity and specificity, it is essential to understand the mechanisms by which the presence of the surrounding environment and analyte affects the properties of the OTFT sensing device. To date, two mechanisms for the detection of analytes by organic semiconductors have been described. First, diffusion of the analyte to the semiconductor−dielectric interface results in a change in the charge-transport behavior owing to the direct interaction of the analyte with the semiconductor channel region. Second, interaction of the analytes with the grain boundaries of the organic semiconductor can affect the electrical signal via the formation of trap and/or dopant states.9,23,25 Both of the above mechanisms are related to interaction of the analyte with the organic semiconductor material in the OTFT. In contrast, interaction of the environment or analyte with the gate dielectric of a bottom-gate OTFT has rarely been considered as a significant contributor to the sensing mechanism of the OTFT device and has been considered passive. Only recently has the impact of the dielectric on the sensing of biological molecules been reported,14 and effects on the OTFT device performance have been shown as a result of a change in the gate dielectric properties (e.g., dielectric constant).26 For the successful fabrication of aqueous sensors based on OTFTs, the impact of each mechanism must be understood to decouple the effect of the environment from that of the analyte. In this study, we use neutron reflectivity (NR) measurements to quantify the film thickness and capacitance measurements to characterize the dielectric properties of cross-linked polymer thin films as a function of immersion in water and 90:10 water− methanol solutions (i.e., using methanol as a model analyte). We demonstrate that immersion results in swelling, which significantly impacts the dielectric properties of the cross-linked polymer thin films and, in turn, the electrical behavior of the devices. The goal of this work is to convey the critical nature of the mechanisms that govern the electrical OTFT signal output in response to the device environment or interaction with a given analyte.
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EXPERIMENTAL SECTION
Materials. All materials were purchased from Aldrich27 and used as received unless otherwise noted. Detailed synthetic procedures for the poly(4-vinylphenol) (PVP)−polystyrene (PS) random copolymers and general analysis of the materials are described in the Supporting 5019
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Figure 1. Reflectivity profiles for (a) neat PVP, (b) 76% PVP−26% PS random copolymer, (c) 50% PVP−50% PS random copolymer, and (d) 22% PVP−78% PS random copolymer. The measurement of the thin films in air was performed with the silicon substrate below the thin film. In contrast, the thin films immersed in water or 90:10 water−methanol solutions were measured in an inverted configuration, with the silicon substrate above the film and water or the water−methanol solution below the film (details are provided in the Supporting Information). The insets in each of (a)−(d) show the reflectivity profiles for the films before exposure to water (air-start) and after the first conditioning cycle (air).
the swelling of the film with water and 90:10 water−methanol solutions. The thicknesses and swelling percentages reported for the thin films denoted as “air-start” and “air” are for thin films measured before exposure to water and after the first conditioning cycle, respectively. The reflectivity profiles for the neat x-PVP thin films in air, water, and a 90:10 water−methanol solution are shown in Figure 1a. The fringe spacing in the reflectivity profile decreases when the neat x-PVP thin film is immersed in water and further decreases when the film is immersed in a water−methanol solution. Such narrower fringe spacings in the reflectivity profile qualitatively are due to an increase in the layer thickness. Modeling of the reflectivity profiles (the fitting procedure and the model fits are shown in the Supporting Information) reveals a 25.5% and a 29.9% increase in thickness when the neat x-PVP thin film is immersed in water and a 90:10 water−methanol solution, respectively. This means that both the media (water) and analyte (methanol) are absorbed by the polymer dielectric and will thus play a critical role in the electronic properties of a sensor device. To study the impact of the swelling on an OTFT device, we evaporated a 40 nm film of an organic semiconductor, DDFTTF, known to be stable during device operation under water,21,23 onto the x-PVP-covered silicon wafer for NR. Swelling of the x-PVP polymer thin film was observed (see Figure S3 and Table S2 in the Supporting Information), as evidenced by a decrease in the fringe spacing when the thin film is immersed in water or 90:10 water−methanol solutions. This shows that an increase in the thickness of the polymer dielectric layer occurs even with the presence of a hydrophobic organic semiconductor top layer, and both water and analyte can
ambient conditions using a two-sourcemeter [Keithley 2635 (DrainSource)/Keithley 2400 (Gate)27] homemade probe station setup.
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RESULTS AND DISCUSSION NR is a powerful technique for characterizing the thin-film structure in the direction perpendicular to the surface.30 In reflectivity, the ratio of the specularly reflected intensity to the incident intensity is measured over a range of incident angles (θ) to yield a reflectivity curve for a thin film. Information about the film thickness, neutron scattering length density profile, and interface roughness can be obtained from modeling of the reflectivity curve (details on the methods used for modeling of the reflectivity curves and the fitted models can be found in the Supporting Information). Neutron radiation has the unique ability to penetrate silicon without prohibitive absorption (as would occur with X-ray radiation), which allows the study of thin films coated on a silicon wafer. Passing the neutron beam through the side of the silicon wafer allows for characterization of the thin films while they are immersed in water. We first measured the thickness of a neat x-PVP layer in air, before immersion in water, with NR and found a thickness of 33.2 nm. Upon immersion in water, the thickness increased to 38.3 nm. However, upon drying of the x-PVP thin film, the thickness was reduced to 30.4 nm. We attribute this decrease in thickness to either a loss of un-cross-linked molecules or rearrangement of the film upon swelling. No further significant changes in the thickness were observed after subsequent conditioning cycles (immersion in water or water−methanol solutions for >3 h, drying under vacuum for >1 h, and equilibration with ambient humidity for >2 h). Thus, we use the film thickness after the first conditioning cycle to compare 5020
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Figure 2. (a) Percentage of swelling in water and a water−methanol solution as a function of the copolymer composition. (b) Dielectric constant in air, water, and a water−methanol solution as a function of the copolymer composition. The confidence intervals shown represent a combination of the uncertainties from the measurement of the device area, the measurement and modeling of the film thickness, and the measured capacitance. Details of the calculation of the dielectric constant and corresponding confidence intervals are provided in the Supporting Information.
clear that increasing the PS content leads to a decrease in the device performance. However, in all cases, the devices are still able to operate at the required Vsource−drain < 1.2 V. It should also be mentioned that the devices were fabricated at identical temperatures for DDFTTF deposition. Thus, it is possible that improved device performance could be achieved via the individual optimization of material deposition conditions. In order to determine the effect of swelling on the dielectric properties of the polymer thin films, we thermally evaporated gold electrodes and measured the capacitance of the resulting device (Scheme 1d). The device capacitances measured in air, immersed in water, and immersed in a water−methanol solution are shown in Figure S4 and Table S2 in the Supporting Information. When immersed in water or water− methanol solutions, a capacitance increase was observed for the cross-linked neat PVP and PVP−PS copolymer thin films. While swelling of the cross-linked polymer dielectric layer leads to an increase in the thickness (reducing the capacitance of the device), an increase in the capacitance due to inclusion of water (which has a higher dielectric constant than the polymer) dominates the overall capacitance change. Using the thickness of the films measured with NR, we calculated the dielectric constant (εr) of the polymer layer in air, water, and a 90:10 water−methanol solution (see Figure 2b). For the neat x-PVP film, the dielectric constant increased from 5.1 in air to 20.9 and 20.7 when immersed in water and water−methanol solutions, respectively, representing an ∼300% increase in the dielectric constant. The introduction of hydrophobic PS into the film leads to a decrease in the change of the dielectric constant upon immersion in an aqueous solution because of a decrease in the swelling of the film. There was no statistical difference in the dielectric constant with immersion in water or water−methanol solutions for each polymer composition. All of the films show at least a 78% increase in the dielectric constant upon immersion in an aqueous solution, demonstrating that the changes in the dielectric constant are significant, even for low PVP content. These results are important because the large changes in the dielectric constant affect the electric field at the semiconductor−dielectric interface and thus significantly alter the electrical signal. Our results show that, upon interpretation of the response of an OTFT sensor, it is critical to understand the interaction of the environment and analyte with each individual component of the device (e.g., active material, dielectric, electrodes). The establishment of such experimental guidelines
significantly permeate through the crystalline organic semiconductor thin film. To reduce the effect of water swelling on the properties of the dielectric thin film, a more hydrophobic dielectric can be chosen. However, typical hydrophobic dielectrics (PS and CYTOP31) have not been compatible with device operation at the ultralow voltages required for underwater sensing (Vsource−drain < 1.2 V).23 To address this issue, we synthesized a series of PVP−PS random copolymers; see Scheme 1 (synthesis details in the Supporting Information). By tuning the composition of the thin film (PVP is hydrophilic, and PS is hydrophobic), we aim to alter the effects of immersing the thinfilm dielectric in an aqueous solution. We used NR to measure the swelling of the cross-linked polymer films when immersed in water and 90:10 water−methanol solutions. These measurements reveal a decrease in the swelling, in both water and a water−methanol solution, with increasing PS content (Figures 1b−d and 2a). Additionally, a similar decrease in the film thickness after the first conditioning cycle is observed, with the magnitude of the thickness decrease becoming smaller as the PS content is increased (see Table S1 in the Supporting Information and the insets in Figure 1). To confirm that the polymers function as stable dielectric materials, we fabricated OTFTs utilizing the organic semiconductor, DDFTTF. In each case, a 40 nm film of the organic semiconductor was thermally evaporated on the polymer substrate held at 60 °C. The temperature was held below 60 °C to ensure that the polymer films remained below the glass transition temperature because previously this has been shown to lead to a sharp decrease in the device performance.2 The device was completed by evaporating 40 nm of gold as the source and drain electrodes. The results of the transistor performances are summarized in Table 1. From the table, it is Table 1. Summary of the OTFT Device Performances Utilizing the Native x-PVP, as Well as the PVP−PS Copolymersa material x-PVP 76% PVP−PS 50% PVP−PS 22% PVP−PS
average μ (cm2/ V·s) 0.178 0.089 0.047 0.023
± ± ± ±
0.015 0.008 0.003 0.002
VT (V) −0.25 −0.25 −0.34 −0.21
± ± ± ±
0.06 0.04 0.05 0.07
on/off ratio 4.0 1.9 8.6 3.2
× × × ×
103 103 102 102
a
Representative transfer curves are shown in Figure S5 in the Supporting Information. 5021
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L.; Kowalewski, T.; Weiss, L. E.; McCullough, R. D.; Fedder, G. K.; Lambeth, D. N. Sens. Actuators, B 2007, 123 (2), 651−660. (5) Lu, G.; Usta, H.; Risko, C.; Wang, L.; Facchetti, A.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130 (24), 7670−7685. (6) Baude, P. F.; Ender, D. A.; Haase, M. A.; Kelley, T. W.; Muyres, D. V.; Theiss, S. D. Appl. Phys. Lett. 2003, 82 (22), 3964−3966. (7) Sokolov, A. N.; Tee, B. C. K.; Bettinger, C. J.; Tok, J. B. H.; Bao, Z. Acc. Chem. Res. 2011, 45 (3), 361−371. (8) Mabeck, J.; Malliaras, G. Anal. Bioanal. Chem. 2006, 384 (2), 343−353. (9) Roberts, M. E.; Sokolov, A. N.; Bao, Z. N. J. Mater. Chem. 2009, 19 (21), 3351−3363. (10) Torsi, L.; Dodabalapur, A. Anal. Chem. 2005, 77 (19), 380. (11) Katz, H. E. Electroanalysis 2004, 16 (22), 1837−1842. (12) Lipomi, D. J.; Vosgueritchian, M.; Tee, B. C. K.; Hellstrom, S. L.; Lee, J. A.; Fox, C. H.; Bao, Z. Nat. Nano 2011, 6 (12), 788−792. (13) Sokolov, A. N.; Roberts, M. E.; Johnson, O. B.; Cao, Y. D.; Bao, Z. A. Adv. Mater. 2010, 22 (21), 2349−2353. (14) Angione, M. D.; Cotrone, S.; Magliulo, M.; Mallardi, A.; Altamura, D.; Giannini, C.; Cioffi, N.; Sabbatini, L.; Fratini, E.; Baglioni, P.; Scamarcio, G.; Palazzo, G.; Torsi, L. Proc. Natl. Acad. Sci. U.S.A. 2012. (15) Macaya, D. J.; Nikolou, M.; Takamatsu, S.; Mabeck, J. T.; Owens, R. M.; Malliaras, G. G. Sens. Actuators, B 2007, 123 (1), 374− 378. (16) Schwartz, G.; Tee, B. C.; Mei, J.; Appleton, A. L.; Kim do, H.; Wang, H.; Bao, Z. Nat. Commun. 2013, 4, 1859. (17) Cotrone, S.; Ambrico, M.; Toss, H.; Angione, M. D.; Magliulo, M.; Mallardi, A.; Berggren, M.; Palazzo, G.; Horowitz, G.; Ligonzo, T.; Torsi, L. Org. Electron. 2012, 13 (4), 638−644. (18) Liao, F.; Yin, S.; Toney, M. F.; Subramanian, V. Sens. Actuators, B 2010, 150 (1), 254−263. (19) Pennington, J. C.; Gunnision, D.; Harrelson, D. W.; Brannon, J. M.; Zakikhani, M.; Jenkins, T. F.; Clarke, J. U.; Hayes, C. A.; Myers, T.; Perkins, E.; Ringelberg, D.; Townsend, D. M.; Fredrickson, H.; May, J. H. Natural Attenuation of Explosives in Soil and Water Systems at Department of Defense Sites. Interim Report of U.S. Army Corps of Engineers, 1999 (20) McCully, K. S. Am. J. Clin. Nutr. 2007, 86 (5), 1563. (21) Roberts, M. E.; Mannsfeld, S. C. B.; Stoltenberg, R. M.; Bao, Z. N. Org. Electron. 2009, 10 (3), 377−383. (22) Yoon, M. H.; Yan, H.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127 (29), 10388−10395. (23) Roberts, M. E.; Mannsfeld, S. C. B.; Queralto, N.; Reese, C.; Locklin, J.; Knoll, W.; Bao, Z. N. Proc. Natl. Acad. Sci. U.S.A. 2008, 105 (34), 12134−12139. (24) Roberts, M. E.; Mannsfeld, S. C. B.; Tang, M. L.; Bao, Z. N. Chem. Mater. 2008, 20 (23), 7332−7338. (25) Crone, B.; Dodabalapur, A.; Gelperin, A.; Torsi, L.; Katz, H. E.; Lovinger, A. J.; Bao, Z. Appl. Phys. Lett. 2001, 78 (15), 2229−2231. (26) Veres, J.; Ogier, S. D.; Leeming, S. W.; Cupertino, D. C.; Khaffaf, S. M. Adv. Funct. Mater. 2003, 13 (3), 199−204. (27) Commercial materials, instruments, and equipment are identified in this paper to specify the experimental procedure as completely as possible. In no case does such identification imply a recommendation or endorsement by the National Institute of Standards and Technology nor does it imply that the materials, instruments, or equipment identified are necessarily the best available for the purpose. (28) Meng, H.; Zheng, J.; Lovinger, A. J.; Wang, B. C.; Van Patten, P. G.; Bao, Z. N. Chem. Mater. 2003, 15 (9), 1778−1787. (29) Nelson, A. J. Phys.: Conf. Ser. 2010, 251, 012094. (30) Anastasiadis, S. H.; Russell, T. P.; Satija, S. K.; Majkrzak, C. F. J. Chem. Phys. 1990, 92 (9), 5677−5691. (31) Sokolov, A. N.; Cao, Y.; Johnson, O. B.; Bao, Z. Adv. Funct. Mater. 2012, 22, 175.
will lead to mechanistic interpretation of the sensor behavior and, thus, is expected to result in the development of more robust and sensitive detection platforms. In conclusion, we combined NR and capacitance measurements to quantify the swelling and dielectric properties of cross-linked polymer thin films when immersed in water and 90:10 water−methanol solutions. We show that swelling and an increase in the dielectric constant are greater for thin films with a higher PVP-to-PS ratio. In order to develop a proper mechanistic understanding of the effect of an analyte upon the signal from OTFT sensors, it is critical to decouple the interactions of the analyte with the semiconducting and dielectric layers. To date, most studies have focused on interactions of analytes with the semiconducting component of OTFTs. Our results emphasize the importance of considering changes in the dielectric properties of the gate dielectric as well during the development of mechanistic models for OTFT sensing, and they can allow the development of methods to control the sensor response by controlling how the sensing environment interacts with the dielectric.
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ASSOCIATED CONTENT
* Supporting Information S
Details of polymer synthesis, NR, and data fitting, capacitance measurements, and dielectric constant calculation. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address ⊥
Department of Chemistry, Bogazici Unıversity, Bebek, Istanbul 34342, Turkey. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the neutron research facilities used in this work. E.V. thanks the Eastman Kodak Corp. and Kodak Fellows Program for support. Z.B. acknowledges partial financial support from the National Science Foundation (Grant NSF ECCS 1101901). Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. A.N.S. acknowledges financial support from the NSF-ECCS-EXP-SA program (Grant NSF ECCS-0730710) and Office of Naval Research (Grant N000140810654). P.W. and D.K. were supported by the Air Force Office of Scientific Research (Grant FA9550-121-0190).
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REFERENCES
(1) Arias, A. C.; MacKenzie, J. D.; McCulloch, I.; Rivnay, J.; Salleo, A. Chem. Rev. 2010, 110 (1), 3−24. (2) Kim, C.; Facchetti, A.; Marks, T. J. Science 2007, 318 (5847), 76− 80. (3) Klauk, H.; Zschieschang, U.; Pflaum, J.; Halik, M. Nature 2007, 445 (7129), 745−748. (4) Li, B.; Santhanam, S.; Schultz, L.; Jeffries-El, M.; Iovu, M. C.; Sauve, G.; Cooper, J.; Zhang, R.; Revelli, J. C.; Kusne, A. G.; Snyder, J. 5022
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